This invention relates to circuit breakers of the type having a bimetallic thermal trip element and, more particularly, to circuit breakers for distribution transformers to control moderate power distribution on feeder circuits.
The disclosed circuit breaker is particularly adaptable for use with distribution transformers. Transformers used in power distribution systems are generally associated with a protective device which prevents or limits current overload damage to the transformer and its associated apparatus. The completely self-protected transformer includes a circuit breaker on the secondary or low voltage side to protect against damage due to overload currents. The secondary breaker disconnects the transformer from its load if the load current becomes dangerously high.
Commonly used circuit breakers incorporate three basic features: (1) a low overload signal device, (2) an incremental increase adjustment and (3) a tripping device to open the contacts of the circuit breaker upon a predetermined overload. As the load current through the circuit breaker increases a low overload point is reached at which the overload indicator signal switch closes and energizes a signal light on the outside of the transformer housing. The signal light which is mounted on the transformer enclosure, provides a visual indication that the secondary circuit breaker is about to trip. That is, the signal light is turned on at a lower overload than that required to trip the circuit breaker, indicating that load current is approaching trip level. The signal light remains on until reset, even though the load current later falls to a satisfactory level. When line crews see an illuminated signal light they are thereby given notice that a moderate overload condition has occurred. Repeated observations of an illuminated signal light usually indicate that the transformer should be replaced with a higher capacity transformer. The signal light also gives a quick and accurate visual indication of where a tripped breaker is located when a power outage occurs. As the load current continues to increase, a second overload point is reached where the circuit breaker trips open. The circuit breaker tripping protects the transformer against severe damage due to the flow of excessive overload currents.
To add extra loadability and minimize customer outage until transformer changeover can be implemented, an incremental load increase adjustment emergency control device is included in the circuit breaker. It is desirable, in many applications, to change the overload capacity of a distribution transformer. In these applications, reducing the transformer life somewhat by sustaining a moderate overload is better than interrupting the load current and causing a power outage. As stated, the completely self-protected transformer includes an emergency control device which effectively changes the rating of the associated circuit breaker. The emergency control mechanism can be moved from its normal position to allow the resetting of a tripped circuit breaker. The rating of the circuit breaker may be increased by the emergency control mechanism for a short period of time until the bimetal-actuated latch in the circuit breaker has cooled sufficiently to allow setting of the rating of the circuit breaker at its normal position.
A problem exists in some of the prior art transformer circuit breakers which use plastic members for the operating and/or support of the circuit breaker. Holding the many dimensions with the associated close tolerances requires extremely accurate molding. The hot oil environment in which the circuit breaker must function accurately is less than ideal for even the best plastics. The retention of the calibration during transformer processing and in service depends to a large extent on the plastic materials maintaining their specified dimensions. Another problem with many prior art transformer circuit breakers is the extensive use of flexible copper leads. In prior art circuit breakers a flexible lead is required entering the breaker at the single stationary contact to provide for contact movement during contact closure. A flexible lead is also required between the moving contact and the bimetal, and a flexible lead is also required to exit the breaker from the other end of the bimetal to allow for movement of the bimetal assembly during calibration and when the breaker is reset. The extensive use of copper braid and the many brazes required to fabricate the circuit breaker are difficult to control and expensive to manufacture. The large number of brazes is required for each prior art circuit breaker yields a higher probability of producing a substandard joint. This can be particularly serious upon joints close to the bimetal since a defective joint here will effect the calibration of the circuit breaker.